Output tubes will either slowly wear out over a couple years, or in some cases they can short.  

When the wear normally, the power decreases and detail decreases.  Best way to find out if they are getting weak is to install a fresh pair and compare the sound.  If you don't hear a difference, reinstall the old pair and try again in six months.

If a tube shorts, it will take out the 1 k 2 watt resistor on the power tube socket inside the amp.  When this happens, you'll either get scratchy sound at a low level from the guilty channel, or silence.  This requires that the amp be sent back so the resistor can be replaced, and the bad tube should be destroyed so as not to be installed in another amp.  

There are 3 versions of the 6N1P. 2 of them have plated pins, 1 does not.  Avoid the ones that do not have plated pins.  Of the two that do, the 6P15P-EV is the military grade, long life tube that we sell.  This tube has been re-branded as Svetlana in the past.  All of them have the floating suppressor grid.

Hope this helps.

Steve